The present invention relates to a heat resistant adhesive composition, in particular, to a modified bismaleimide in admixture with modified polyamideimide for heat resistant adhesives with high performance.
In general, the adhesives used in three-layer flexible printed circuit (3-layer FPC) or three-layer tape automatic bonding (3-layer TAB) are of acrylic or epoxy series. The glass transition temperature (Tg) and heat resistance of the acrylic or epoxy series are lower than those of substrates, i.e., polyimide (PI). Thus, with increasing requirements of high packaging density for electronic devices, the adhesives having low Tg and poor temperature resistance are limited in the manufacturing process and cannot be widely applied in adhesion of electronic devices required for high density.
Owing to the fact that bismaleimide (BMI) possesses heat resistance which is about the same as that of polyimide and is reactive, BMI can be used to improve the high temperature resistance of adhesives for 3-layer FPC or 3-layer TAB. However, due to the crosslinking reaction of BMI, the product obtained after the crosslinking reaction is relatively hard and fragile. That is, BMI is not suited for direct use in the adhesives. In order to get better softness and to be consistent with the requirements for 3-layer FPC or 3-layer TAB, BMI should be improved or modified. After being modified, BMI is admixed with modified polyamide-imide (PAI), the composition will form adhesives with heat resistance, such as disclosed in Japan Patent No. Sho-61-98782. However, the above composition used as adhesives in 3-layer FPC or 3-layer TAB still results in drawbacks in processing. For example, after coating the PI substrate with BMI, the BMI coating hardens and shrinks, generally resulting in poor surface smoothness on the final products.
Japan Patent No. Sho-61-98782 discloses an invention of utilizing PAI to react with an epoxy resin in reducing the number of unreacted PAI functional groups and then adding a suitable amount of diamine modified BMI to improve the heat resistance and the adhesion with metals.
The present invention and Japan Patent No. Sho-61-98782 all teach the BMI/PAI series for a heat resistant adhesive composition. However, in the Japan patent, the BMI of the BMI/PAI series is modified by diamine. When the modified BMI coats and hardens onto the PI substrate, owing to the reactivity of diamine itself, the crosslinking reaction of the double bond often occurs. The coating film will shrink considerably and result in a poor degree of surface smoothness in the final products. On the other hand, the BMI used in the present invention is modified by special synthesis technology and is then mixed with PAI, which is modified by epoxy in producing a heat resistant adhesive composition. The Tg value of the adhesive composition will not decrease. The shrinkage is small and the dimension of the coating film is constant. Most importantly, the properties of heat resistance and adhesion are good.
To eliminate the drawbacks of poor surface smoothness as with the Japanese art cited herein, a special synthesis technology is utilized in this invention to adequately modify the quality of BMI by free radical reaction. First, the double bond of BMI is reacted, that is, the monomer of BMI is modified and reacts to become oligomers which have higher molecular weights and can be dissolved in solvent. Subsequently, the above oligomers are mixed with PAI, which has been modified by epoxy, to form the desired composition. However, the modification of BMI itself relies on the reaction of the double bond thereof by free radical reaction. For example, after the BMI coats and hardens on the substrate, the shrinkage for the improved BMI decreases while still retaining excellent adhesive properties with the substrate. Owing to the hardening reaction for the double bond of BMI itself, the heat resistance of the above modified BMI is better than that of BMI modified by diamine. Furthermore, the Tg of the modified BMI based on the reaction of double bonds can be kept at a high value. However, if BMI is modified by diamine, the value of Tg will decrease considerably and the dimensional stability will decrease.
Although there are other patents relate to adhesive compositions of BMI/PAI systems, the BMI of these patents is not modified by barbituric acid (BTA) and the ratios of the compositions are also different from that of the present invention. Hence, the present invention provides an adhesive composition with excellent surface smoothness and high heat resistance properties for 3-layer FPC or 3-layer TAB.